The present invention relates to a receiver at the head-end or side in a communications system or network. More specifically, the invention relates to a receiver for signals in the upstream direction which is the direction from user to head-end or a centralizing unit that is linked to a number of users, the number being equal to or larger than one.
The technology area of communications systems is subject to a vast engineering effort in order to allow for an always increasing number of applications.
Currently, the public access network for television (CATV) is being prepared for bi-directional communication. The goal of upgrading the CATV network is to provide bi-directional communication of digital data at speeds well beyond that of traditional data communication over telephone lines. This way, CATV data communication allows new types of applications such as video-on-demand and fast Internet access. It also provides an alternative to existing telephone services. In that case, the analog voice data is digitized and transmitted as a collection of data packets. For CATV communications, one defines the downstream direction, going from the broadcasting head-end side to the user side, and the upstream direction, going from the user side to the broad-casting head-end side. These definitions of upstream and downstream direction in the sequel are adopted also for any communications system or communications network. The upstream direction is defined as going from a user side or a subscriber residence to a central office or head-end or a centralizing unit that is linked to a number of users, the number being larger than one.
The communication process between the end-user and head-end is typically organized as a number of hierarchical subprocesses, each running at a different level of abstraction. This hierarchy is needed to express the access of the communications network between communicating parties in an efficient way. This invention is concerned with the communication subprocess on the lowest hierarchical layer, i.e., the physical layer.
The goal of communications on the physical layer is:
to provide a reliable means of data communication by applying methods of data modulation and demodulation, and
to transfer this reliable communication method towards the higher layers. This is done by means of an interface and a protocol.
The topology of the physical layer a typical public access network for television is shown in FIG. 1. Both the head-end side and user side have a transmitter and receiver to make bi-directional communication possible. The network has a tree-like topology, and consists of both active elements (bi-directional amplifiers) and passive elements (cable, splitters and taps). For each head-end transmitter/receiver pair, many user-end transmitter/receiver pairs may exist. Typically, 400 users can be served through 1 head-end. The up- and downstream communications path run over a single electrical path and are differentiated through frequency multiplexing, as shown in FIG. 2.
U.S. Pat. Nos. 3,962,637 and 5,127,051 describe improved modems for high frequency data transmission. In particular, U.S. Pat. No. 5,127,051 discloses a modem system that can adapt to fast channel variations by rapidly deriving accurate channel estimate without excessive storage of data overhead. Accordingly, the data are transmitted in a frame carrying at least two identical data sequences. This approach, however, has as a disadvantage a major overhead for static type of channel and therefore has a slow performance for these channel types. The publication xe2x80x9cCATV Return Path Characterization for Reliable Communicationsxe2x80x9d by C. A. Eldering, IEEE Communications Magazine, August 1995 addresses the problem of reliable solutions for bi-directional communication. In said publication, an emphasis is given to the key problem of the understanding of the communication channel characteristics in the upstream direction. One of the problems in upstream communication is as follows. As we move from the user to the head-end, the physical transmission medium (the cable or the channel) is shared by an increasing amount of users. Therefore, users will share the medium in the upstream direction by means of an appropriate multi-access protocol. This invention is concerned with the time-division multi-access protocol which uses burst-mode signals. In this protocol, each user gets in turn connection to the head-end during a fixed time-slot. The start and end of the time-slot is decided at the head end by allocation algorithms running in the higher hierarchical communication layers. A discussion of these allocation algorithms is out of the scope of this patent application since it is concerned with the transport of data on the physical layer only.
The key problems to solve in order to establish a reliable upstream communication between the user and the head-end are the following:
1. As the signals emitted at the user side propagate through the upstream channel, they are attenuated and delayed. This attenuation and delay is different for each user, since each user is connected at a different position in the tree network as seen from the head-end. Therefore the head-end must estimate these modulation distortions on a per-burst basis. It must also do this as fast as possible, since during estimation time, no useful data can be transmitted.
2. Besides attenuation and delay, the signals also suffer from group delay distortion. Group delay distortion is caused by the non-linear phase characteristics of the (mainly active) components located in the up-stream channel. The effect of group delay distortion is that the time-domain shape of the distorted signal is changed. The distortion is a linear effect, which means that it can be removed by passing the received signal through a proper filter before detecting it. The required shape of this filter is dependent on the amount and type of group delay distortion, and is again different for each user. Therefore, the head-end receiver must estimate the coefficients of this filter on a per burst basis. Failure to do so causes an effect at the head-end receiver side called inter-symbol-interference (ISI). ISI degrades the quality of the data detection process, and therefore should be avoided.
The process of estimating attenuation, delay and group delay distortion is jointly called channel estimation.
3. The upstream communications CATV path is also susceptible of noise influences. These can be caused by electrical appliances or spurious emissions of radio-band users (mobile communication, amateur, CB, . . . ), and other, unknown sources. Since the actual time-domain shape of noise is unknown, it cannot be removed at the receiver. It will therefore also degrade data detection performance of the receiver. The transmission can however be protected against noise influences by applying a proper encoding of the data. The encoding increases the redundancy of the transmitted data pattern. At the receiver side, the removal of this redundancy can then be used to identify locations of errors in the received data pattern. Eventually, the redundancy can even be used to correct the errored values.
4. All estimation processes active in the head-end receiver must proceed as fast as possible. During the estimation the actual delay, attenuation and group delay distortion is unknown and no data can be detected successfully. The signal transmitted by one user is of a bursty nature. Therefore, the shorter the estimation time, the more time will be left in the signal burst that can be used for the transmission of actual data.
In the remainder of this document, we will first summarize the key properties of the invention, which is a digital receiver for these burst mode signals. Next, we will give a detailed description of the receiver and its operation.
The present invention relates to a receiver at the head-end or a centralizing unit side in a communications system or network. The receiver is adapted for receiving signals in the upstream direction which is the direction from user to head-end or a centralizing unit that is linked to a number of users, the number being equal to or larger than one. The present invention further relates to communication systems making use of burst-mode signals.
The present invention relates to a telecommunication system with means for upstream communication from a user to a head-end over a channel, said means for upstream communication including a receiver comprising a detect unit being configured in a feed-forward data driven architecture.
Each algorithm is executed by a dedicated digital hardware machine, comprising a local controller and a data path. The data path executes the data processing operations inside the algorithm, while the local controller performs operation sequencing, and algorithm synchronization.
In an aspect of the present invention, said detect unit is adapted for receiving a burst-mode signal, said burst-mode signal having a preamble with at least one training portion at the beginning of the burst followed by at least one timing alignment portion.
Said signal further can comprise a user message.
In another aspect of the invention, the detect unit of the system can comprise a block for extracting information on at least one transmission characteristic of said burst-mode signal in said channel, said information being obtained as the coefficients of a fractionally spaced feed-forward equalizer in said block.
The receiver can further comprise a timing block wherein said alignment portion is processed, said alignment portion providing the transition from said training portion to said user message as the downsampling phase of said block.
Yet the receiver can comprise a detection block for detecting said signal and adjusting the power level of said signal to a predetermined power level; and a filter block with programmable coefficients for filtering said user message.
Said coefficients can be extracted from said preamble in real-time.
Yet in a further aspect of the invention, the receiver has a feed-forward architecture that is configured as a chain of subsequent components, said signal being consecutively passed and without feedback through said chain, the chain comprising components having a finite state machine and a data path, the signal being passed through the data paths, the finite state machines running a control program, said components behaving differently when a burst signal is received or not.
Yet the present invention is also related to, in a communication system, a method for transmitting a signal, said method comprising the steps of:
transforming said signal into a first sequence of digital data;
prepending a predetermined sequence of data to said first sequence of data, said predetermined sequence having a training portion at the beginning of the predetermined sequence followed by a timing alignment portion, the sequence of said predetermined sequence and said first sequence being a resulting data sequence; and
modulating said resulting data sequence to a predetermined format for transmission.
The method can further comprise the step of receiving said signal in a receiver with an equalizer block with programmable coefficients, said step comprising the substeps of:
fixing said coefficients while analyzing said training portion of said predetermined sequence of first data; and
detecting said timing alignment portion as the transition to said first sequence of data; and thereafter
performing data filtering on said first sequence of data.
The method can be executed in real-time.
In a further aspect of the invention, a method of operating an adaptive modem for analyzing signals being transmitted over a communications channel is disclosed. Said signals are being sent in at least one burst comprising a preamble and a user message, said method comprise the steps of:
receiving the transmitted signals;
generating a plurality of coefficients for a downsampling feed forward adaptive equalizer from a training sequence in said preamble of said burst;
adapting said downsampling feed forward adaptive equalizer to said communications channel.
According to the method of the invention, the signals are analyzed on a burst-by-burst base and in real-time.
The receiver of the invention is suited for the reception of burst-mode signals. The receiver performs a channel estimation on a per-burst basis in real time or essentially immediate. The channel estimation is necessary to do successful data detection of modulated data.
Current state of the art modems do not perform per-burst channel estimation and/or group delay distortion estimation, but rather assume a fixed channel from which the data can be detected by means of a fixed data filter.
The receiver of the invention performs the channel estimation and data detection in one compact all-digital mechanism that has no tuning parts. The reception method works in an aspect according to the principle of a matched filter receiver, but stores no local copy of the required matched waveform. Rather, a copy of the matched waveform is included in the preamble of the signals.
The burst-mode signals that are received comprise two parts: A preamble and a payload. The preamble has among other functions, the function to perform the channel estimation and synchronize the demodulation loops, while the payload contains the actual data to transmit, including error correcting codes. The preamble comprises at least one training portion followed by a timing alignment portion. In case several training portions are included in the preamble, the method of the invention can be implemented as an averaging algorithm, averaging the results obtained from the different training portions.
The overhead of the burst-mode signal is therefore primarily located in the preamble, since it contains no user data. The present invention contains a very short and fixed preamble.
The channel estimation method allows compensation of an arbitrary amount of group delay distortion through a very simple extension of the burst preamble.
The channel estimation is solely based on the burst preamble and is therefore very fast.
Due to the use of a training sequence, no differential encoding/decoding of the symbols is required, as is traditionally seen on QAM type modems.
The reception method allows reception of different pulse shapes with one single implementation. When the pulse shapes are of the so-called root-raised-cosine (RRC) family (which is the most commonly used shape in state-of-the art quadrature modulated modems), different RRC roll-off factors are supported by one and the same architecture. Other pulse shapes are possible as long as they have the ISI-free property. This property is discussed in the sequel.
In general, the reception method implies that one and the same receiver can be used to receive different standards (self-adapting).
It uses a feed-forward architecture. For a burst-mode receiver, this is an important property. A feed-forward architecture is self-controlled and pipelineable. Due to the absence of feedback loops, system stability is independent of the delay of individual components.
The receiver and method of the invention furthermore in other aspects allow for an immediate channel estimation and allow to compensate for substantially any group delay distortion.
The receiver and method of the invention in yet another aspect allow for combined timing and phase estimation. The receiver of the invention also in an aspect is adapted for receiving signals of transmitters having different characteristics as being for instance the products of different companies or having different roll-off factors such as determined in the DAVIC standard or the IEEE standard or the MCNS standard.